Regulation of high frequency induction heating apparatus



July 9, 1963 A. D. GIACCHETTI REGULATION OF HIGH FREQUENCY INDUCTION HEATING APPARATUS Filed May 5, 1960 R O T A L M C 5 O AMPLIFIER TO O-SCILLQTOR 54 j ILRTO RF. AMP.

.4 i i 1 4442 T INVENTOR. .Qifim cc%eZZ2 BY X W 2 i OSCILLATOR United States Patent f 3,097,283 REGULATION OF HIGH FREQUENCY INDUCTION HEATING APPARATUS Anacleto D. Giacchetti, Chicago, IlL, assignor to National Video Corporation, Chicago, Ell. Filed May 5, 1960, Ser. No. 27,174 10 Claims. (Cl. 21910.77)

This invention relates to induction heating wherein a metallic piece part is subjected to the electromagnetic field resulting from passing electric current through a conductor, usually in the form of a coil, termed the heating coil. In general the piece part is positioned within the secondary of a transformer consisting of one or several turns whereby a comparatively high current, and consequently, a strong electromagnetic field is obtained. To facilitate the heating effect on the object being processed, i.e. the flow of eddy currents, the current in the heating coil is caused to alternate a high frequency, usually on the order of radio frequencies. In particular the invention has reference to apparatus for regulating the magnitude of the current in the heating coil as the same varies due to fluctuations in supply voltage.

It will be recognized that variation in the supply voltage to the high frequency oscillator will affect the current available in the heating coil fed by the output thereof and that such variation results in corresponding variation of the predetermined temperature at which the work peice is processed, with consequent rejects.

This invention has for its principal object the provision of automatically-functioning apparatus for regulating the supply voltage to the oscillator in response to variation in a predetermined value of magnetic field strength, viz., the temperature at which a selected work piece is processed.

Another object is to provide, in voltage regulating means as aforesaid, a standard of reference upon which regulation is made to depend which may be inexpensively fabricated, by the use of which the range, or limits of regulation may be easily calibrated, and which may be interchangeable with other reference standards in accordance with the characteristics of the work piece.

Other objects and advantages of the invention will become apparent from the ensuing description which, taken with the accompanying drawing, discloses certain forms which the invention may assume in practice.

In this drawing:

FIG. 1 is a somewhat schematic illustration of apparatus incorporating the principles of the invention;

FIG. 2 is an enlarged, perspective view of the reference standard of FIG. 1;

FIGS. 3 and 4 show modified arrangements for regulating the voltage applied to the oscillator; and

FIG. 5 shows a modified form of the invention.

Broadly regarded the invention has application to apparatus for heating of a conductor, usually a metallic article which is placed within the alternating magnetic field of a heating coil carrying a heavy, high frequency, alternating current. In general the term heating coil refers broadly to that portion of the output of the oscillator which provides the magnetic field and need not be a coil in the ordinary sense. Where, herein, this portion of the invention is termed a coil the same is intended in the foregoing broad sense.

In principle, the apparatus comprises a vacuum tube oscillator fed from 60 cycle mains through a rectifier providing high frequency energy on the order of radio carrier frequencies, say from 50 kc. to several megacycles. The high frequency output may be taken off the tank circuit in various Ways, for example, by directly tapping the tank inductance or by inductively relating another 3,097,283 Patented July 9, 1963 ICC coil to the tank inductance which may then feed the heating coil or the heating coil may be the secondary of a transformer fed directly or inductively from the tank inductance. In any case the heating coil is arranged to draw a heavy current of a magnitude sulficient to bring the work piece to the pre-sclected temperature. In any case the heating coil is designed with respect to the configuration of the work piece for most efficient utilization of the energy contained in the magnetic field. Even though the apparatus may be initially adjusted to produce the desired heating action variation in the line voltage is inevitable and the net result, where such variation exceeds a certain allowable percentage, may be rejection of the piece parts.

In accordance with the invention the heating coil is connected in series or parallel with a second heating coil within the field of which there is a reference standard comprising a metallic member together with a thermolectric device for converting heat into a control signal, e.g. a thermocouple. This signal is fed to an electronic amplifier, the output of which is utilized to regulate the input to the oscillator through some suitable means, either of the modulating type, the on and off type or by switching between various voltages obtained through a voltage divider.

Thus by initial calibration, based on acceptable heating of the work piece, the reference standard and the circuit which is responsive thereto serve to regulate the output of the oscillator. It will become apparent that the reference standard need not be equivalent in configuration and material to the work piece since the appropriate relation can always be initially established for the various configurations and materials which characterize the Work piece by choice of control circuit parameters and calibration thereof.

Turning then to the drawing (FIG. 1) I have shown, by way of example, typical apparatus incorporating the invention comprising an oscillator 10 having input leads 11 and 12. In practice, the oscillator is provided with a power supply fed from 60 cycle mains. However, the invention is not to be regarded as so limited. The control provided within the present concept may be applied before or after the power supply. For conciseness of exposition and simplicity in the drawing, the leads 11 and 12 are shown as ground and B as from a rectifier but, within the scope of the invention such leads are to be regarded broadly as the input to the oscillator from whatever means derived.

The heating coil of the oscillator within which the work piece is positioned is shown .at 14 and is shown preferably connected in series with an auxiliary heating coil 15. However, a parallel connection may be used, as in FIG. 5. Both coils, as will be understood carry current of a magnitude suficient to provide a magnetic field to heat the work piece to a predetermined temperature which is maintained for some predetermined time, whereafter the work piece is removed from the magnetic field or the oscillator output is interrupted. Usually the latter is the case with the time cycle automatic, being initiated manually and interrupted automatically.

Exemplificatively I have shown a work piece in the form of a getter 21 for a cathode ray tube including a neck 22. The getter is ordinarily supported on the electron gun (not shown). It will be understood that socalled flashing of the getter results in interaction of the active material of the getter with residual gases and vapors in the evacuated tube whereby the degree of vacuum is substantially improved. Inasmuch as the getter is within the tube neck and the tube has been evacuated, induction heating is the recognized method of flashing the getter, i.e. inducing its gas-absorbing action. It will be understood that, for optimum gettering, the getter shall be flashed at an optimum temperature and that such temperature shall remain uniform from tube to tube. Variation in supply voltage will, obviously, represent variation in the temperature at which the getter is flashed.

Installed within the auxiliary or second heating coil 15 is a reference standard or what, for convenience, may be referred to as a dummy load 24 and a temperature-responsive device 26 (FIG. 2). The dummy load may be of any suitable form or material but preferably is an annulus of tantalum, this shape being selected in order that current induced therein may circulate in the optimum fashion, i.e. a short-circuited single turn subject to the alternating flux field. Tantalum is preferred because its vaporization temperature is above the usual temperature whichthe annulus will assume in practice. Any metal or metal alloy of similar characteristics may be employed. In one working embodiment of the invention the annulus is supported in any suitable manner, preferably in an evacuated envelope '27, by a rod 28 seized in a press 29 formed in the well-known manner after the envelope has been pumped down and pinched off. If desired the envelope may be provided with its own getter 31 for the reason explained hereinabove. By maintaining the reference standard in vacuo oxidization is inhibited and pro tection afforded. In any case the reference standard is thermally distinct from the work piece, i.e. is not in heatconducting relation therewith; the principal desideratum being that the reference standard be in magnetic relation with the field of the auxiliary coil 15 in order to sense variation in the current producing the respective magnetic fields in the coils 14 and 15.

Positioned adjacent or contiguous to the dummy load 24. is a thermoelectric device 26, e.g. a junction thermocouple, the leads 33 of which are connected to the input of an electronic or equivalent amplifier 35.

As thus far constituted it will have become apparent that both the work piece and the reference standard are raised to their respective temperatures simultaneously when the oscillator is switched on. In order that variation of voltage derived from the reference standard will provide proportional regulation of the input voltage to the oscillator calibration means may be availed of. Inasmuch as the same may be effected in various ways wellknown in the art of thermometry elucidation is regarded as redundant. Accordingly, for purposes of this specification, it may be assumed that varying voltage from the thermocouple 26 is amplified in the amplifier 35 to provide a continuous variation in some device actuated by the output of the amplifier, and adapted to vary the supply voltage. As one example, I have shown a sensitive relay 41 actuated at some defined current flow in the output of the amplifier. The tongue 42 of the relay 41 is effective, when pulled in, to operate a power relay 44, the winding whereof is in series circuit with the tongue 42, its front contact 45 and a source of voltage 4 7. The tongue 48 and its front contact 49 of the relay 44 are in series with one of the leads feeding the oscillator, say the lead 1.1, whereby the oscillator may be turned on and off when the current supplied to the heating coils 14 and 15 is respectively too low or too high.

However since voltage fluctuation in the oscillator supply may sometimes occur at a rapid rate a relay arrangement as just described may not be the most acceptable. Under this circumstance a modulating type of con trol may be found preferable.

For example, in FIG. 3, I have shown a solenoid 51 connected into the output of the amplifier having a core 52 adapted to operate a potentiometer 54 through suitable mechanism, eg a rack 56 secured to the core 52 and a pinion 57. Thus continuous variation in the feed to the oscillator may be derived from the varying output of the amplifier. Equivalent arrangements are possible, e.g.

a wafer switch connected to taps of a voltage divider whereby regulation would occur as a step-by-step change in oscillator supply voltage or an infinitely variable transformer such as the Variac type may be used. In either event the mechanical linkage will include the core of the solenoid.

FIG. 4 shows another arrangement wherein two B+ voltages are made available and operation of the relay 44a is effective to switch therebetween. Thus, one such voltage may represent the maximum tolerable voltage which will yield proper heating of the work piece and is represented by the position of the tongue 48a shown, and therefore normal operation of the apparatus. Upon drop in supply voltage to the oscillator the tongue 48a moves against its back contact and the higher of the two 13-],- voltages is switched in to restore the apparatus to normal operation or an acceptable substantially equivalent condition.

As an accessory feature an indicating type of thermometer or a recording pyrometer may be added to provide a visual indication, or permanent record, of the performance of the apparatus. Since such instruments are well known, and their connection to the thermocouple 2 6 or the amplifier circuit therefor follow common practice detailed treatment thereof is deemed unnecessary.

While I have shown particular embodiments of my invention, it will be understood, of course, that I do not Wish to be limited thereto since many modifications may be made and I, therefore, contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

I claim:

1. In induction heating apparatus of the type wherein high frequency alternating current through a heating coil fed by the output end of an oscillator is utilized to establish a magnetic field to which a work piece is subjected for raising the same to a selected temperature and a circuit is provided for supplying operating voltage to the oscillator, the improvement which comprises: an auxiliary heating coil connected to the oscillator output adapted to provide a second magnetic field, a reference standard of metallic material permanently positioned in the second magnetic field to be heated concurrently with the work piece, said reference standard being thermally distinct from said work piece; means to sense the temperature of the reference standard, and means under the control of said sensing means connected in, the oscillator supply circuit to vary the supply voltage in response to varying temperature of said reference standard.

2. In induction heating apparatus of the type wherein high frequency alternating current through aheating coil fed by the output of an oscillator is utilized to establish a magnetic field to which a work piece is subjected for raising the same to a selected temperature and a circuit is provided for supplying operating voltage to the oscillator the improvement which comprises: a second coil connected in series with the heating coil and the output end of the oscillator, a reference standard positioned in the magnetic field to be heated concurrently with the work piece, said reference standard being positioned with respect to said workpiece to preclude conduction of heat from the workpiece to the reference standard, means to sense the temperature of the reference standard, and means under the control of said sensing means connected to the oscillator supply circuit to vary the supply voltage in response to varying temperature of said reference standard.

3. The improvement in accordance ,with claim 1 wherein said auxiliary heating coil is in series connection with the first mentioned heating coil and the oscillator output end.

4. The mprovement in accordance with claim 1 wherein said auxiliary heating coil is in parallel connection with the first mentioned heating coil.

5. The improvement in accordance with claim 1 wherein said reference standard comprises paramagnetic material and said sensing means is a thermoelectric element contiguous to said standard.

6. The improvement in accordance with claim 5 wherein said paramagnetic material is characterized by a temperature of vaporization substantially higher than the temperature which the reference standard normally assumes.

7. The improvement in accordance with claim 5 further characterized in that said reference standard and thermoelectric element are enclosed in an evacuated envelope.

8. In induction heating apparatus of the type wherein high frequency alternating current through a heating coil fed by the output end of an oscillator is utilized to establish a magnetic field to which a work piece is subjected for raising the same to a selected temperature and a circuit is provided for supplying operating voltage to the oscillator the improvement which comprises: a second heating coil in series connection with the first mentioned heating coil across the oscillator output end, a reference standard positioned for inductive heating by the magnetic field of said second heating coil, a thermoelectric element to sense the temperature of the reference standard, and means responsive to the voltage of said element connected to the oscillator supply circuit to increase and decrease the input voltage to the oscillator in accordance with decrease and increase in the temperature sensed by said element.

9. The improvement in accordance with claim 8 wherein said reference standard is a paramagnetic annulus positioned with its diametrical plane perpendicular to the principal direction of the magnetic field of the second coil produced by said second heating coil.

10. The improvement in accordance with claim 9 further characterized in that said annulus and thermoelectric element are enclosed in an evacuated envelope.

References Cited in the file of this patent UNITED STATES PATENTS 2,041,029 Stargardter May 19, 1936 2,647,983 Boyd Aug. 4, 1953 2,724,037 Bock Nov. 15, 1955 2,773,161 Baker Dec. 4, 1956 

1. IN INDUCTION HEATING APPARATUS OF THE TYPE WHEREIN HIGH FREQUENCY ALTERNATING CURRENT THROUGH A HEATING COIL FED BY THE OUTPUT END OF AN OSCILLATOR IS UTILIZED TO ESTABLISH A MAGNETIC FIELD TO WHICH A WORK PIECE IS SUBJECTED FOR RAISING THE SAME TO A SELECTED TEMPERATURE AND A CIRCUIT IS PROVIDED FOR SUPPLYING OPERATING VOLTAGE TO THE OSCILLATOR, THE IMPROVEMENT WHICH COMPRISES: AN AUXILIARY HEATING COIL CONNECTED TO THE OSCILLATOR OUTPUT ADAPTED TO PROVIDE A SECOND MAGNETIC FIELD, A REFERENCE STANDARD OF METALLIC MATERIAL PERMANENTLY POSITIONED IN THE SECOND MAGNETIC FIELD TO BE HEATED CONCURRENTLY WITH THE WORK PIECE, SAID REFERENCE STANDARD BEING THERMALLY DISTINCT FROM SAID WORK PIECE; MEANS TO SENSE THE TEMPERATURE OF THE REFERENCE STANDARD, AND MEANS UNDER THE CONTROL OF SAID SENSING MEANS CONNECTED IN THE OSCILLATOR SUPPLY CIRCUIT TO VARY THE SUPPLY VOLTAGE IN RESPONSE TO VARYING TEMPERATURE OF SAID REFERENCE STANDARD. 